1. Field of the Invention
The present invention relates to a high frequency narrow-band multi-mode filter which comprises surface acoustic wave (hereinafter, referred to as SAW) resonators or surface skimming bulk wave (hereinafter, referred to as SSBW) resonators and which exhibits a narrow band pass characteristic of between several tens of megahertz and one gigahertz.
2. Description of the Prior Art
Band pass filters for frequencies above 50 MHz include an LC filter, a spiral filter, a SAW filter or a band pass filter in which resonance elements or inductors in a basic circuit are replaced with SAW resonators.
However, such band pass filters have the following defects.
In the case of the LC and spiral filters, they both have a low Q value. In particular, the spiral filter is large in size. In the case of the SAW filter, on the other hand, it has a relatively wide band and a large insertion loss. In addition, there occur ripples in its pass band due to triple transit echo (TTE).
Finally, in the case of the band pass filter of the type where resonance elements are replaced with SAW resonators, this type of filter is the same as a conventional crystal filter in principle. However, when it is to be operated for high frequencies above 100 MHz, it has been very difficult to set or fix the resonance frequencies of the elements and differences therebetween with high accuracy.